1. Field of the Invention
This invention relates to a digital signal transmission system for transmitting serially the digital data signals with a large volume of information such as, for example, pulse-code modulated audio-signals.
2. Description of the Prior Art
Generally, when performing such operations as recording, playback, editing, etc., by converting the audio signals of music, etc., into PCM (pulse coded modulation), it is required to make data transmission between the plural units such as PCM recorder, electronic editing machine, specific effect generator such as a digital reverberator. In this case, it is undesirable to perform D-A convergence or A-D convergence for each unit because there takes place pulse-code modulation (PCM) due to signal deterioration upon every convergence to reduce the effect provided from an improvement of the S/N. Therefore, data transmission is conducted in the form of digital signals such as PCM signals.
Here, pulse-code modulation of audio signals of music, etc., is briefly explained. The analog signals which are continuous in time and amplitude, such as audio signals, are sampled by picking them up with a sampling pulse with a constant period and the amplitude of the sampled signals is changed into a debunching amplitude to effect so-called quantization, and the value of the quantized amplitude is encoded by, for example, binary code to form the PCM signals. For said sampling pulse, there is selected, for example 44 KHz or 50 KHz. According to said binary code, one sample is given with, for example, 16 bits per one word.
For parallel transmission of such digital signals (for example, parallel transmission of 16 bits of one word), it needs to connect 16 signal transmission lines such as twisted-pair transmission lines, and in case of transmitting the PCM audio signals of plural channels, for example 4 channels, it is required to connect as many as 64 signal transmission lines. This makes wiring of the transmitting and receiving terminals of the associated units very complicated and also tends to lower reliability of the connections.
If the digital signals such as PCM audio signals are transmitted serially with a single signal transmission line, the line connecting work is simplified and also its reliability is enhanced. In this case, generally the data reading clock signals are sent separately, but because of extremely large volume of information per unit time, the clock frequency for reading each bit in said data, even in the case of 16 bits in one word, is about 0.7 Mbit/sec. when the sampling frequency is 44 kHz and about 0.8 Mbit/sec. when the sampling frequency is 50 kHz. It is also required to provide in one word not only the audio signal data but also unit control bits or user's bits for free use by the user, so that it is desirable to adopt a 32-bits-in-one-word capacity to give full scope. In this case, if the sampling frequency is supposed to be 50 kHz, it is necessary to transmit digital signals of about 1.6 Mbit/sec., so that when clock synchronization is made between the transmitting and receiving sides, even about 100 m of transmission becomes impossible even if a deviation of half wavelength of the clock signals is permitted. Further, in case the data are reciprocated between the master units (such as electronic editing machine) and slave units (such as PCM recorder), the transmitting distance becomes even shorter substantially.
Out of the 32 bits in one word of such digital signal, for example 20 bits are assigned for sampling data and 12 bits for control, but such number of control bits may become insufficient in case the control information or user information has increased. If the number of bits in one word is increased, however, the bit clock frequency is further enhanced, and this necessitates speed-up of circuit response, making it difficult to effectuate clock synchronization.